Lewis acid induced [4+3] cycloadditions of 2-silyloxyacroleins. Insights on the mechanism from a DFT analysis

The mechanism for the Lewis acid induced [4+3] cycloadditions of 2-(trimethylsilyloxy)acrolein with furan has been examined here through DFT calculations at B3LYP/6-31G* level. The mechanism is a three-step process initialized by the nucleophilic attack of furan to the β-conjugated position of acrolein yielding a zwitterionic intermediate. The key step on the formation of the seven-membered ring is the electrophilic attack of the furan residue to the carbonyl carbon in this intermediate. The endo selectivity experimentally observed is reproduced by the calculations.